修复演示恢复默认模板覆盖逻辑

- 新增后端默认模板服务，集中维护腹腔镜胆囊切除术和头颈部CT分割的权威分类树、颜色、maskid 和层级定义。

- 演示恢复出厂设置时强制恢复系统默认模板，缺失模板会重建，已修改或删减的默认语义分类树会覆盖回默认状态。

- 清理 main.py 中重复的默认模板定义，让启动 seed 复用同一套服务逻辑，避免后续默认模板定义漂移。

- 扩展管理员恢复出厂设置测试，覆盖头颈部CT模板被改坏和腹腔镜模板缺失后的恢复结果。

- 更新 AGENTS、README 和需求/API/测试/前端审计文档，明确恢复出厂设置会权威恢复系统默认模板。

backend/tests/test_admin.py

@@ -184,7 +184,27 @@ def test_demo_factory_reset_leaves_admin_and_parsed_demo_dicom(client, db_sessio
     assert db_session.query(ProcessingTask).count() == 0
     assert db_session.query(Template).filter(Template.owner_user_id.is_not(None)).count() == 0
     preserved_templates = db_session.query(Template).filter(Template.owner_user_id.is_(None)).all()
-    assert [template.name for template in preserved_templates] == ["头颈部CT分割"]
+    templates_by_name = {template.name: template for template in preserved_templates}
+    assert set(templates_by_name) == {"腹腔镜胆囊切除术", "头颈部CT分割"}
+    head_neck_classes = templates_by_name["头颈部CT分割"].mapping_rules["classes"]
+    lap_classes = templates_by_name["腹腔镜胆囊切除术"].mapping_rules["classes"]
+    assert [item["name"] for item in head_neck_classes] == [
+        "肿瘤/结节 (Tumor/Nodule)",
+        "下颌骨 (Mandible)",
+        "甲状腺 (Thyroid)",
+        "气管 (Trachea)",
+        "颈椎 (Cervical Spine)",
+        "颈动脉 (Carotid Artery)",
+        "颈静脉 (Jugular Vein)",
+        "腮腺 (Parotid Gland)",
+        "下颌下腺 (Submandibular Gland)",
+        "舌骨 (Hyoid Bone)",
+        "待分类",
+    ]
+    assert [item["maskId"] for item in head_neck_classes] == [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 0]
+    assert len(lap_classes) == 36
+    assert lap_classes[-1]["name"] == "待分类"
+    assert lap_classes[-1]["maskId"] == 0
     assert db_session.query(AuditLog).count() == 1
     assert db_session.query(AuditLog).first().action == "admin.demo_factory_reset"